Design of a rotor-tied doubly fed induction generator

[摘要] ENGLISH ABSTRACT: The severe e ects of climate change has seen a global push towards an increase in the use of renewable energy sources. Wind energy is a major renewable energy source and its usagein electricity generation is steadily rising. The doubly fed induction generator (DFIG) is the most commonly used generator in wind turbines due to its wide speed range of operation and easy power factor control implementation. DFIGs also use fractionally rated power converters which have lower converter losses compared to fully rated converters in induction and synchronous generators.Recently, a new DFIG topology (the rotor-tied DFIG) has been proposed. In this topology, the DFIG's rotor is connected to the grid and the stator to fractionally rated power converters. This topology has been shown to lead to higher efficiency as the higher frequency (grid frequency) is on the rotor core, which is typically smaller than the stator core, thus lower core losses. It was also suggested that designing DFIGs in this topology could lead to higher power densities. The topology requires no extra complexity in operations as a similar control system used with a conventional DFIG can be used.The steady state operation of the rotor-tied DFIG is first discussed and a new method of calculating slip for this topology is given. The proportions of the rotor and stator power to the input mechanical power to the generator at different slip values are illustrated. The transformer model equivalent circuit of rotor-tied DFIGs is also described.The purpose of this study is to design and optimize a low power rotor-tied DFIG. The design process is presented in a sequential manner from the calculation of the rotor size, to the rotor and stator winding parameters, then the slot and core dimensions. The obtained model is then evaluated with infinite element analysis (FEA) specific for rotor-tied DFIGs.The FEA is used to evaluate the power density, power factor and efficiency. The harmonic content in the model is also assessed. The design is then optimized to increase the power density and lower the harmonic content.A 5.5 kW rotor-tied DFIG is designed and its performance is evaluated using FEA. The optimization is executed using a response surface approximation of the FEA model with a genetic algorithm and this significantly reduces optimization time. The design of experimentsfor the response surface approximation is based on a combination of the latin hypercube sampling and composite sampling methods.Finally, a prototype is constructed and tested in a DFIG standalone mode. The tests are conducted in the sub-synchronous, synchronous, and super-synchronous regions of operation. The stator is excited with DC at synchronous speed, and slip frequency AC by the use of an AC drive at other speeds. The rotor is connected to a resistive load in all the tests. Tests results show that the prototype's efficiencies at synchronous and sub-synchronous speeds, for the rated stator current, are similar. The efficiencies at super-synchronous speeds are however lower with the same rated stator current due to the power rating of the AC drive used to excite the stator.

[发布机构] Stellenbosch University